1. Field of the Invention
This invention relates to moisture separator reheaters, and more particularly, to the relative shell side and tube side fluid flow directions and relative placement of steam reheating means.
2. Description of the Prior Art
Light water nuclear reactors generally produce saturated or only slightly superheated steam of relatively low pressure. During the process of expanding steam in a steam turbine and extracting mechanical energy therefrom, the moisture percentage in the steam increases as it passes through the steam turbine. For most fossil fueled power cycles moisture forms only in the relatively late turbine stages in the vicinity of the turbine exhaust since fossil fueled power cycles generally utilize throttle steam of relatively high pressure and significant superheat. Turbines used in conjunction with nuclear fueled power cycles experience substantial moisture formation in many of the turbine stages due to the relatively low pressure, saturated throttle condition for the steam. Such moisture in nuclear steam turbines can impair the efficiency of turbine blades subjected to it and cause them to erode at excessive rates. For such reasons, it is highly desirable to remove the moisture from the moist steam before further expansion of that steam through the turbine. Furthermore, it has been found that reheating the steam after its separation from the moisture provides further increases in the efficiency of the power cycle.
It has been common practice to combine moisture separators with steam reheaters in a single shell member. A further common practice has been to provide stages of reheat in series flow relation across the path of the relatively cool cycle steam. Such staging has often been accomplished by providing separate tube bundles each of which is supplied with a different steam temperature with the steam temperatures increasing in the direction of cycle steam flow. Such low and high pressure reheating tube bundles have been disposed in both the horizontal and vertical series flow positions, but both dispositions have commonly utilized reheater inlet pass tubes arranged vertically over reheater outlet pass tubes in fluid communication therewith. Prior art has commonly utilized cycle steam inlets on the end of and cycle steam outlets on the top of the shell enclosure member. Also known in the prior art is the use of disposing cycle steam inlets on one sidewall of the shell member and situating cycle steam outlets on an opposite side of the shell member. Such a scheme is disclosed by U.S. Pat. No. 3,923,009, issued Dec. 2, 1975. That invention's horizontal disposal of the cycle steam inlets and outlets drastically reduced the shell side pressure loss and provided a simultaneous increase in the cycle efficiency over those cycles which previously utilized moisture separator reheaters having cycle steam inlets disposed on the moisture separator reheater's ends.
A problem encountered in prior art moisture separator reheaters included of the heating fluid subcooling on the interior of the reheater tubes. Such subcooling was caused by relatively cold cycle inlet steam contacting the exterior of the heating fluid's tubes in the outlet or other secondary pass. At some point within the outlet or secondary pass tubes, heating fluid therein was entirely condensed and any further heating of the cycle steam contacting the tube's exterior had to originate from subcooling the already condensed heating fluid flowing therein. Such subcooling caused the inlet and outlet pass fluid heating tubes to have different tube temperatures, resulting in differential thermal expansion therebetween. Such differential thermal expansion resulted in high weld stresses and caused tube binding when U tubes were used. The previously mentioned patent does not reduce the magnitude of such subcooling since the exterior of the outlet or other secondary pass fluid heating tubes are still subjected to the coldest cycle steam in the moisture separator reheater. Such temperature difference tends to extract additional heat from the heating fluid which, when all has been condensed, must be subcooled below its condensing temperature. To combat the subcooling problem, the prior art has utilized such schemes as orifice plates on radially inner inlet pass tubes to force greater portions of heating fluid into the radially outer inlet pass tubes which are in fluid communication with the radially outer outlet pass fluid heating tubes. The use of such orifice plates, however, has disadvantages in the cost of materials, cost of assembly within the moisture separator reheater unit, and increased tube side pressure drop.
Interceptor valves are commonly located on the cycle steam outlets of moisture separator reheaters and are periodically tested by opening and closing. On previous "end" cycle steam inlet designs shell erosion and tube vibration were problems during interceptor valve testing since, in some cases, twice the maximum rated cycle steam flow passed through the shell and across the tube bundles. With the present invention, however, the full shell length and cross-section are available for steam flow with the steam flow through the optional separators and across the tube bundles being nearly the same as that existent during full load flow.
On previous designs the low pressure and high pressure tube bundles were assembled in series flow relation on their shell side. When it became necessary to remove one or more of the low pressure tube bundles from service, the steam whch was normally heated by that bundle would pass through without any heating and be mixed with the balance of the steam which was heated. Such failure to reheat a portion of the cycle steam can have a substantial effect on cycle efficiency.
A further disadvantage of the end inlet, top outlet design is that the piping required thereby and the piping required by many of the new side entry turbines are not compatible.
As can be seen, most prior art problems stem directly from one or more of the following steam reheater characteristics: cycle steam end inlet-top outlet configuration, series flow relation between low pressure-high pressure tube bundle within the same shell member, and the vertical tube side traverse of the heating fluid in such manner that the outlet pass tubes on each bundle are exposed on their shell side to the coolest cycle steam within the moisture separator reheater unit.